delay documentation and checkpatch additions

new file mode 100644
index 0000000..12fcb7e
@@ -0,0 +1,97 @@
+delays - Information on the various kernel delay / sleep mechanisms
+-------------------------------------------------------------------
+
+This document seeks to answer the common question: "What is the
+RightWay (TM) to insert a delay?"
+
+This question is most often faced by driver writers who have to
+deal with hardware delays and who may not be the most initimately
+familiar with the inner workings of the Linux Kernel.
+
+
+Inserting Delays
+----------------
+
+The first, and most important, question you need to ask is "Is my
+code in an atomic context?"  This should be followed closely by "Does
+it really need to delay in atomic context?" If so...
+
+ATOMIC CONTEXT:
+	You must use the *delay family of functions. These
+	functions use the jiffie estimation of clock speed
+	and will busy wait for enough loop cycles to achieve
+	the desired delay:
+
+	ndelay(unsigned long nsecs)
+	udelay(unsigned long usecs)
+	mdelay(unsgined long msecs)
+
+	udelay is the generally preferred API; ndelay-level
+	precision may not actually exist on many non-PC devices.
+
+	mdelay is macro wrapper around udelay, to account for
+	possible overflow when passing large arguments to udelay.
+	In general, use of mdelay is discouraged.
+
+NON-ATOMIC CONTEXT:
+	You should use the *sleep[_range] family of functions.
+	There are a few more options here, while any of them may
+	work correctly, using the "right" sleep function will
+	help the scheduler, power management, and just make your
+	driver better :)
+
+	-- Backed by busy-wait loop:
+		udelay(unsigned long usecs)
+	-- Backed by hrtimers:
+		usleep(unsigned long usecs)
+		usleep_range(unsigned long min, unsigned long max)
+	-- Backed by jiffies / legacy_timers
+		msleep(unsigned long msecs)
+		msleep_interruptible(unsigned long msecs)
+
+	Unlike the *delay family, the underlying mechanism
+	driving each of these calls varies, thus there are
+	quirks you should be aware of.
+
+
+	SLEEPING FOR "A FEW" USECS ( < ~10us? ):
+		* Use udelay
+
+		- Why not usleep?
+			On slower systems, (embedded, OR perhaps a speed-
+			stepped PC!) the overhead of setting up the hrtimers
+			for usleep *may* not be worth it. Such an evaluation
+			will obviously depend on your specific situation, but
+			it is something to be aware of.
+
+	SLEEPING FOR ~USECS OR SMALL MSECS ( 10us - 20ms):
+		* Use usleep_range
+
+		- Why not msleep for (1ms - 20ms)?
+			Explained originally here:
+				http://lkml.org/lkml/2007/8/3/250
+			msleep(1~20) may not do what the caller intends, and
+			will often sleep longer (~20 ms actual sleep for any
+			value given in the 1~20ms range). In many cases this
+			is not the desired behavior.
+
+		- usleep vs usleep_range:
+			Since usleep is built on top of high-resolution timers,
+			you will trigger an interrupt almost *exactly* when your
+			sleep expires; normally, sleeps (by their nature) do not
+			need this kind of precision. The *much* friendlier
+			usleep_range allows the kernel to complete your sleep
+			any time in the given range, likely when some other
+			interrupt has already woken up the kernel for some other
+			reason.
+
+	SLEEPING FOR LARGER MSECS ( 10ms+ )
+		* Use msleep or possibly msleep_interruptible
+
+		- What's the difference?
+			msleep sets the current task to TASK_UNINTERRUPTIBLE
+			whereas msleep_interruptible sets the current task to
+			TASK_INTERRUBTIBLE before scheduling the sleep. In
+			short, the difference is whether the sleep can be ended
+			early by a signal. In general, just use msleep unless
+			you know you have a need for the interruptible varient.

index bd88f11..e3625ac 100755
@@ -2570,6 +2570,14 @@ sub process {
 			}
 		}
 
+# prefer usleep_range over udelay
+		if ($line =~ /\budelay\s*\((.+)\);/) {
+			# ignore udelay's < 10, however
+			if (! (($1 =~ /(\d+)/) && ($1 < 10)) ) {
+				WARN("usleep_range is preferred over udelay; see Documentation/timers/delays.txt\n" . $line);
+			}
+		}
+
 # warn about #ifdefs in C files
 #		if ($line =~ /^.\s*\#\s*if(|n)def/ && ($realfile =~ /\.c$/)) {
 #			print "#ifdef in C files should be avoided\n";

index e3625ac..0650ab9 100755
@@ -2578,6 +2578,11 @@ sub process {
 			}
 		}
 
+# prefer usleep_range over usleep
+		if ($line =~ /\busleep\s*\(.+\);/) {
+			WARN("usleep_range is preferred over usleep; see Documentation/timers/delays.txt\n" . $line);
+		}
+
 # warn about #ifdefs in C files
 #		if ($line =~ /^.\s*\#\s*if(|n)def/ && ($realfile =~ /\.c$/)) {
 #			print "#ifdef in C files should be avoided\n";

index 0650ab9..bbc2e76 100755
@@ -2583,6 +2583,13 @@ sub process {
 			WARN("usleep_range is preferred over usleep; see Documentation/timers/delays.txt\n" . $line);
 		}
 
+# warn about unexpectedly long msleep's
+		if ($line =~ /\bmsleep\s*\((\d+)\);/) {
+			if ($1 < 20) {
+				WARN("msleep < 20ms can sleep for up to 20ms; see Documentation/timers/delays.txt\n" . $line);
+			}
+		}
+
 # warn about #ifdefs in C files
 #		if ($line =~ /^.\s*\#\s*if(|n)def/ && ($realfile =~ /\.c$/)) {
 #			print "#ifdef in C files should be avoided\n";